1. Field of the Invention
This invention relates to a radiation image readout method for exposing a stimulable phosphor sheet carrying a radiation image stored therein to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the radiation energy stored, and photoelectrically detecting the emitted light by use of a photodetector.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-ray, cathode rays or ultra-violet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly for diagnostic purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored in the stimulable phosphor varies over a very wide range in proportion to the amount of energy stored therein, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor to the radiation by reading out the emitted light with an appropriate read-out gain and converting it to an electric signal to reproduce a visible image on a recording medium or a display device. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. This is very advantageous in practical use.
As mentioned above, in the radiation image recording and reproducing system using a stimulable phosphor sheet, compensation for deviation of the level of the radiation energy stored in the stimulable phosphor sheet from a desired level can easily be carried out by adjusting the read-out gain to an appropriate value when photoelectrically reading out the light emitted from the stimulable phosphor sheet upon stimulation thereof. Therefore, the quality of the reproduced radiation image is not adversely affected by a fluctuation in radiation dose due to fluctuating tube voltage or MAS value of the radiation source, a variation in the sensitivity of the stimulable phosphor sheet or the photodetector, a change in radiation dose according to the condition of the object, or a fluctuation in the radiation transmittance according to the object, and the like. Also, it is possible to obtain a desirable radiation image even when the radiation dose to the object is low. Further, it is possible to obtain a radiation image having a high image quality of high contrast, high sharpness and low noise, and the like, by converting the light emitted from the stimulable phosphor sheet into an electric signal, and processing the electric signal as desired. Particularly, when the radiation image is used for medical diagnosis, it is possible to obtain a radiation image processed in the manner most suitable for a particular portion of the human body such as the heart, the chest or the like, thereby realizing an improvement in diagnostic efficiency and accuracy.
However, in order to eliminate various influences caused by the fluctuation of radiographic exposure conditions and/or to obtain a radiation image having a high image quality or a high diagnostic efficiency and accuracy, it is necessary to investigate the radiation image information stored in the stimulable phosphor sheet, for example, image input conditions, or the image input pattern which is determined by the portion of the body (for example, the chest or the abdomen) or the image recording method used, such as plain image recording or contrasted image recording, before final image read-out is conducted for obtaining an electric image signal for use in reproduction of a visible image for viewing, particularly for diagnostic purposes, and then to adjust the read-out conditions, for example, the read-out gain, or the image processing conditions to appropriate values on the basis of the detected radiation image information. It is also necessary to determine the scale factor to optimize the resolution in accordance with the contrast of the image input pattern.
One method of grasping the radiation image information stored in the stimulable phosphor sheet prior to the final read-out is proposed, for example, in U.S. Pat. No. 4,578,581. In the method, preliminary read-out for grasping the radiation image information is conducted by uniformly exposing the whole surface of the stimulable phosphor sheet to stimulating rays and detecting the amount of light emitted by the whole surface of the stimulable phosphor sheet prior to the final read-out for obtaining an electric image signal which is used for reproducing a visible image for viewing, particularly for diagnostic purposes.
On the other hand, in a method disclosed, for example, in Japanese Unexamined Patent Publication No. 58(1983)-67240, the preliminary read-out for grasping the radiation image information is conducted by scanning the stimulable phosphor sheet carrying a radiation image stored therein by stimulating rays of a level lower than the level of the stimulating rays used in the final read-out and detecting the light emitted by the stimulable phosphor sheet during the scanning. The method proposed in Japanese Unexamined Patent Publication No. 59(1984)-19939 is advantageous over the method disclosed in Japanese Unexamined Patent Publication No. 58(1983)-67240 in that the radiation image information stored in the stimulable phosphor sheet can be grasped simply and quickly. However, the former has the drawback described below.
Namely, even when an image of the same portion of the object is recorded under the same radiation exposure conditions, the amount of the light emitted by the whole surface of the stimulable phosphor sheet will differ if the size of the stimulable phosphor sheet or the area thereof actually exposed to X-rays is different. Therefore, when read-out conditions and/or image processing conditions are adjusted on the basis of the detected light amount, the read-out conditions and/or image processing conditions become different in accordance with a change in the size of the stimulable phosphor sheet or a change in the area thereof actually exposed to X-rays.